The study of the reinnervation of the adult goldfish optic tectum by regenerating axons is being continued. The numbers of axonal processes present during various stages of regeneration will be directly assessed by making counts of axons in cross sections of optic tracts at various periods after optic tract section. Several approaches will be used in an effort to evaluate the role of glial cells in the guidance of regenerating axons in the optic tectum. Initially the proliferative pattern of glial cells will be studied using H3-thymidine labelling. EM radioautographic methods will be used to trace the process of synaptogenesis. The response of the mammalian retinal ganglion cells, which do not regenerate, will be compared to goldfish cells which do. Optic tracts will be cut in hooded rats, the animals will be killed 2 days to 2 months later and the morphological changes in the retinal ganglion cells will be assessed at the EM level and compared to the response of goldfish cells. The effects of hyperinnervation on histogenetic cell death will be studied in the rat. When visual cortex is removed at birth, the retinal axons hyperinnervate the tectum and the lateral nucleus of the optic tract, resulting in both hypertrophy and hyperplasia of these target nuclei. The possibility that the increased numbers of retinal fibers decrease the amount of histogenetic cell death will be investigated using morphometric and H3-thymidine - labelling methods. BIBLIOGRAPHIC REFERENCES: Murray, M., H. Jones, H.F. Cserr, and D.P. Rall. 1975 The blood-brain barrier and ventricular system of Myxine glutinosa. Brain Research 99: 17-33. Murray, M. 1976 Regeneration of Retinal Axons into Goldfish Optic Tectum. J. Comp. Neurol. In press.